1. Field of the Invention
This invention relates generally to thermal regenerators and more particularly to a method of forming a thermal regenerator from a thin strip of material having sufficient thermal conductivity to form the heat transfer surfaces of the regenerator, and also having spacers on the strip that neither nest nor are compressed.
2. Description of the Related Art
Many devices, Stirling cycle machines in particular, include a thermal regenerator to which thermal energy is transferred from a flowing fluid, and from which thermal energy is transferred to the fluid. Regenerators are normally made with large surface area structures, such as wool, foil or spheres, and are made of metal, such as stainless steel, or another suitable material that absorbs thermal energy but does not conduct it especially well.
In a Stirling cycle engine, for example, a working gas is moved between a warmer space and a cooler space by a reciprocating displacer to drive a reciprocating piston. The gas is heated during one part of the cycle, and cooled During another part. When the warm gas is being transported from the warmer space, it flows through a regenerator, and thermal energy is transferred to the regenerator by convection, i.e., the impingement of heated gas molecules on the regenerator""s surfaces. The regenerator is warmed and the gas is cooled when thermal energy is transferred to the regenerator as the gas flows through the regenerator to the cooler space.
Once the gas has been cooled in the cooler space, it is driven again through the regenerator; ordinarily in the opposite direction as when the gas was driven from the warmer space. The cooler gas flowing through the regenerator is warmed by the same convection mechanism by which the gas warmed the regenerator: impingement of gas molecules on the regenerator""s surfaces. Regenerators therefore improve the efficiency of the Stirling cycle engine because the gas enters the heated end pre-warmed, and gas enters the cooler end pre-cooled. Of course, regenerators improve the efficiency of many machines other than Stirling cycle machines.
In conventional regenerators, there must be a substantial amount of contact between the flowing fluid molecules and the surfaces of the regenerator in order for substantial heat transfer to occur. One type of regenerator used in Stirling cycle machines uses a long thin strip of metal, such as stainless steel, that is wound up in a roll and placed in a chamber through which gas flows longitudinally of the roll. Each xe2x80x9clayerxe2x80x9d of the metal has a space or gap between it and the next adjacent xe2x80x9clayerxe2x80x9d for fluid to pass through.
It is desirable to have uniform spacing of the layers of such a wound regenerator, but it is often difficult, in practice, to achieve such uniformity of spacing. Localized deformations, such as xe2x80x9cbumpsxe2x80x9d, can be formed on the strip of metal that is subsequently wound to form a regenerator. These bumps, each of which has a corresponding cavity on the opposite side of the strip, can accurately and inexpensively space one wound xe2x80x9clayerxe2x80x9d of the strip from another, in order to promote uniform gas flow through the regenerator.
In conventional methods for making wound regenerators, the bumps can be compressed and crushed if the strip is wound too tightly onto a spool structure. Additionally, bumps of one layer may nest within cavities formed on an adjacent layer of the strip, thereby defeating, at least partially, the advantageous effect of the bumps.
Non-uniform gaps result in high fluid flow rates through larger gaps, and low flow rates through smaller gaps. Non-uniform flow is disadvantageous, because large gaps permit some gas flowing through the regenerator to make poor contact with the surfaces with which thermal transfer should take place, and small gaps restrict the flow of gas therethrough. Furthermore, the pressure drop that is critical to the class of machines referred to as free-piston machines is often compromised with conventional regenerators, thereby resulting in unanticipated dynamic motion of the moving parts.
There is therefore a need for a method of making a wound regenerator that maintains substantially uniform spacing of the layers of a wound regenerator throughout the entire region of the regenerator through which fluid flows.
The invention is a method of producing a wound roll, such as a regenerator, from an elongated strip. The method comprises forming a plurality of spacers on the strip by deforming the strip in a plurality of discrete locations along the strip""s length. Another step of the method includes positioning elongated wires on the strip near opposing lateral strip edges. A further step includes winding the strip and the wires around a rotating take-up spool, thereby forming strip layers where portions of the strip are wound over previously wound portions of the strip with wires interposed between adjacent layers of the strip. The wires are removed from the wound strip, such as by pulling them from between the strip layers. Preferably, the wires have a diameter substantially equal to a spacer height, and the spacers of each strip layer seat against an adjacent strip layer, thereby spacing the layers from one another and leaving uniform thickness gaps extending entirely through the regenerator.
In an alternative embodiment, the method comprises extending the elongated strip through a forming tool and winding the elongated strip around a rotatable take-up spool downstream of the forming tool. Furthermore, the method includes rotating the take-up spool through a predetermined angle that is a fraction of a complete rotation of the take-up spool, thereby advancing the elongated strip through the forming tool a predetermined distance that is a function of the predetermined angle. Next, the take-up spool is stopped and then the forming tool is actuated to deform the strip locally to form at least one spacer on the strip.
The steps of rotating the take-up spool, stopping the spool and actuating the forming tool are repeated until the take-up spool has been rotated about 360 degrees. Once the take-up spool has been rotated about 360 degrees, the take-up spool is rotated through the predetermined angle plus an offset angle to advance the elongated strip through the forming tool a distance that is different from the predetermined distance, such as more or less than the predetermined distance. The take-up spool is then stopped and then the forming tool is actuated to deform the strip locally to form at least one spacer on the strip.
The above steps of advancing the take-up spool, stopping and actuating the forming tools are repeated for a plurality of complete rotations of the take-up spool. The process forms layers of the elongated strip where a portion of the elongated strip is wound around the take-up spool over a previously wound portion of the elongated strip. The offset angle is added to the predetermined angle in order to offset the spacers that are formed in adjacent layers, thus inhibiting alignment of spacers on adjacent layers.
In one embodiment, the method described immediately above also includes positioning at least one elongated wire on the strip upstream of the take-up spool and then winding the strip and said at least one wire around the take-up spool with said at least one wire interposed between adjacent layers of the strip. Subsequently, the wire is removed.
The spacers can be bumps formed in the strip by plastically deforming the strip, such as by forcing the foil into a recess with a molded tool, thereby stretching the foil locally. The tips of each of the bumps seat against the next adjacent layer of the strip, thereby spacing each layer uniformly from the next adjacent layer. The spacers can be tabs.